One document matched: draft-floyd-dcp-ccid2-03.txt
Differences from draft-floyd-dcp-ccid2-02.txt
Internet Engineering Task Force
INTERNET-DRAFT Sally Floyd
draft-floyd-dcp-ccid2-03.txt Eddie Kohler
ICIR
24 May 2002
Expires: November 2002
Profile for DCCP Congestion Control ID 2:
TCP-like Congestion Control
Status of this Document
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of [RFC 2026]. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
This document contains the profile for Congestion Control
Identifier 2, TCP-like Congestion Control, in the Datagram
Congestion Control Protocol (DCCP) [DCCP]. DCCP implements a
congestion-controlled, unreliable flow of datagrams suitable
for use by applications such as streaming media. The TCP-like
Congestion Control CCID is used by senders who are able to
adapt to the abrupt changes in the congestion window typical
of the AIMD (Additive Increase Multiplicative Decrease)
congestion control in TCP. TCP-like Congestion Control is
particularly useful for senders who would like to take
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advantage of the available bandwidth in an environment with
rapidly changing conditions.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . 4
1.1. Usage Scenario . . . . . . . . . . . . . . . . . . . 4
1.2. Example Half-Connection. . . . . . . . . . . . . . . 5
2. Connection Establishment. . . . . . . . . . . . . . . . 6
3. Congestion Control on Data Packets. . . . . . . . . . . 6
4. Acknowledgements. . . . . . . . . . . . . . . . . . . . 7
4.1. Congestion Control on Acknowledgements . . . . . . . 7
4.1.1. Derivation of Ack Ratio Decrease. . . . . . . . . 8
4.2. Quiescence . . . . . . . . . . . . . . . . . . . . . 9
4.3. Acknowledgements of Acknowledgements . . . . . . . . 9
5. Explicit Congestion Notification. . . . . . . . . . . . 10
6. Relevant Options and Features . . . . . . . . . . . . . 10
7. Application Requirements. . . . . . . . . . . . . . . . 10
8. Thanks. . . . . . . . . . . . . . . . . . . . . . . . . 10
9. References. . . . . . . . . . . . . . . . . . . . . . . 10
10. Authors' Addresses . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This document contains the profile for Congestion Control Identifier
2, TCP-like Congestion Control, in the Datagram Congestion Control
Protocol (DCCP).
DCCP uses Congestion Control Identifiers, or CCIDs, to specify the
congestion control mechanism in use on a half-connection. (A half-
connection might consist of data packets sent from DCCP A to DCCP B,
plus acknowledgements sent from DCCP B to DCCP A. DCCP A is the HC-
Sender, and DCCP B the HC-Receiver, for this half-connection. In
this document, we abbreviate HC-Sender and HC-Receiver as "sender"
and "receiver", respectively.)
The TCP-like Congestion Control CCID sends data using a close
variant of TCP's congestion control mechanisms. It is suitable for
senders who can adapt to the abrupt changes in the congestion window
typical of AIMD (Additive Increase Multiplicative Decrease)
congestion control in TCP, and particularly useful for senders who
would like to take advantage of the available bandwidth in an
environment with rapidly changing conditions.
The congestion control mechanisms described here closely follow
mechanisms standardized by the IETF for use in TCP. We do not define
these mechanisms anew; instead, we rely on existing TCP
documentation. This is both to avoid respecifying TCP, and to allow
our specification to track TCP as it evolves. Conformant CCID 2
implementations may actually track TCP's evolution directly, as
updates are standardized in the IETF, rather than waiting for
revisions of this document. CCID 2 does define an additional
mechanism not currently standardized for use in TCP, namely
congestion control on acknowledgements as achieved by the Ack Ratio.
Also, DCCP is a datagram protocol, so several parameters whose units
are bytes in TCP, such as the congestion window cwnd, have units of
packets in DCCP.
For simplicity, we refer to DCCP-Data packets sent by the sender,
and DCCP-Ack packets sent by the receiver. Both of these categories
are meant to include piggybacked DCCP-DataAck packets.
1.1. Usage Scenario
TCP-like Congestion Control is intended to provide congestion
control for the flow of data packets from the server to the client
for applications that do not require fully reliable data
transmission, or that desire to implement reliability on top of
DCCP. TCP-like Congestion Control is appropriate for flows that
would like to receive as much bandwidth as possible over the long
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term, consistent with the use of end-to-end congestion control, and
that are willing to undergo the halving of the congestion window in
response to a congestion event.
1.2. Example Half-Connection
This example, taken from the main DCCP draft, is of a half-
connection using TCP-like Congestion Control specified by CCID 2.
Again, the "sender" is the HC-Sender, and the "receiver" is the HC-
Receiver.
(1) The sender sends DCCP-Data packets, where the number of packets
sent is governed by a congestion window cwnd, as in TCP. Each
DCCP-Data packet uses a sequence number. The sender also sends
an Ack Ratio feature option specifying the number of data
packets to be covered by an Ack packet from the receiver.
(2) The receiver sends a DCCP-Ack packet acknowledging the data
packets for every Ack Ratio data packets transmitted by the
sender. Each DCCP-Ack packet uses a sequence number and
contains an Ack Vector. Because DCCP does not use reliable
transfer, the DCCP-ACK packet does not have a Cumulative
Acknowledgement field.
(3) The sender continues sending DCCP-Data packets as controlled by
the congestion window. Upon receiving DCCP-Ack packets, the
sender examines the Ack Vector to learn about marked or dropped
data packets, and adjusts its congestion window accordingly.
Because this is unreliable transfer, the sender does not
retransmit dropped packets.
(4) Because DCCP-Ack packets use sequence numbers, the sender has
direct information about the fraction of loss or marked DCCP-Ack
packets. The sender responds to lost or marked DCCP-Ack packets
by modifying the Ack Ratio sent to the receiver.
(5) The sender acknowledges the receiver's acknowledgements at least
once per congestion window. If both half-connections are
active, the sender's acknowledgement of the receiver's
acknowledgements is included in the sender's acknowledgement of
the receiver's data packets. If the reverse-path half-
connection is quiescent, the sender sends a DCCP-DataAck packet
that includes an Acknowledgement Number in the header.
(6) The sender estimates round-trip times and calculates a TimeOut
(TO) value much as the RTO (Retransmit Timeout) is calculated in
TCP. The TO is used to determine when a new DCCP-Data packet
can be transmitted when the sender has been limited by the
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congestion window and no feedback has been received from the
receiver.
(7) Each DCCP-Data packet is sent as ECN-Capable with either the
ECT(0) or the ECT(1) codepoint set, as described in [ECN NONCE
DRAFT]. For DCCP-Data packets from the sender, the receiver
returns the ECN Nonce in the DCCP-Ack packet. The DCCP-Ack
packets from the receiver are sent as ECN-Capable with ECT(0).
For DCCP-Ack packets from the receiver, the sender observes
directly if the CE codepoint is set in the received DCCP-Ack
packet.
2. Connection Establishment
Use of the Ack Vector is MANDATORY on CCID 2 half-connections, so
the sender MUST send a `Change(Use Ack Vector, 1)' option to the
receiver as part of connection establishment. The sender SHOULD NOT
send data until it has received the corresponding `Confirm(Use Ack
Vector, 1)' from the receiver.
3. Congestion Control on Data Packets
The data sender uses the congestion window cwnd to control the
sending of packets, and uses the slow-start threshold ssthresh to
control adjustments to cwnd. These integer parameters have units
measured in packets. When halved, their values are rounded down,
except that neither parameter is ever less than one. The cwnd and
ssthresh variables are modified as in TCP. The initial window is
determined using the specification for TCP. The equivalent of a TCP
MSS is simply one packet.
The sender uses the information in Ack Vectors to infer a lost
packet. Ack Vectors explicitly declare which packets have not yet
been received. One of these packets, P, is inferred to be lost
(rather than delayed) when at least NUMDUPACK packets after packet P
have been acknowledged by the receiver. The NUMDUPACK parameter
equals 3, the number of duplicate acknowledgements TCP requires to
infer a loss. A congestion event is defined as one or more packets
lost or marked from a window of data. For each congestion event,
cwnd is halved, then ssthresh is set to the new cwnd. Cwnd is never
reduced below one packet.
When cwnd < ssthresh, meaning that the sender is in slow-start, the
congestion window is increased by one packet for every DCCP-Ack
packet received acknowledging a new DCCP-Data packet from the
sender. Note that cwnd is increased by one per DCCP-Ack received,
not by one per packet acknowledged by the DCCP-Ack; this follows
TCP's behavior. When cwnd >= ssthresh, the congestion window is
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increased by one packet for every window of data acknowledged
without lost or marked packets.
If all of the data packets from a window of data are lost, the
sender needs timeouts to know when to send a new data packet. The
sender estimates the round-trip time at most once per window of
data, and uses the TCP algorithms for maintaining the average round-
trip time, mean deviation, and timeout value. Because DCCP does not
retransmit data, DCCP does not require TCP's recommended minimum
timeout of one second. After a timeout, the slow-start threshold is
set to cwnd/2, then cwnd is set to one packet, and a new packet is
transmitted (thus using up cwnd). The exponential backoff of the
timer is used exactly as in TCP.
4. Acknowledgements
This section describes how the receiver reports acknowledgement
information back to the sender. DCCP-Ack packets from the receiver
MUST include Ack Vector options, as well as an Acknowledgement
Number acknowledging the most recent packet received from the
sender. Acknowledgement data in the Ack Vector options SHOULD
generally cover the receiver's entire Unacknowledged Window, as
described in the DCCP draft.
The sender specifies the Ack Ratio to be used by the receiver. In
the absence of congestion on the reverse path, the Ack Ratio is set
to two if the congestion window is three or more packets, and is set
to one otherwise. The receiver sends a DCCP-Ack packet for every
Ack Ratio packets sent by the sender.
4.1. Congestion Control on Acknowledgements
In CCID 2, the acknowledgement subflow is loosely congestion-
controlled by the Ack Ratio specified by the sender. The receiver
sends (cwnd / Ack Ratio) acknowledgement packets for each window of
data packets. We note that CCID 2 differs from TCP, which presently
has no congestion control for pure acknowledgement traffic. For
congestion control for the pure ack stream, DCCP does not try to be
TCP-friendly, but just tries to avoid congestion collapse, and to be
somewhat better than TCP, in terms of reducing the ack sending rate
in the presence of a high packet loss or marking rate on the return
path.
There are three constraints on the Ack Ratio. First, it is always
an integer. Second, it is never greater than half the congestion
window (with fractions rounded up). Third, it is at least two for a
congestion window of four or more packets.
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DCCP-Ack packets from the receiver contain sequence numbers, so the
sender can infer when DCCP-Ack packets are lost. The sender
considers a DCCP-Ack packet lost if at least NUMDUPACK packets with
higher sequence numbers have been received from the receiver.
(Again, NUMDUPACK equals 3.) If DCCP-Ack packets from the receiver
are marked in the network, the sender sees these marks directly.
DCCP responds to congestion events on the return path by modifying
the Ack Ratio, loosely emulating TCP. For each congestion window of
data with lost or marked DCCP-Ack packets, the Ack Ratio is doubled,
subject to the constraints noted above. Similarly, if the Ack Ratio
is R, then for each (cwnd/(R^2 - R)) congestion windows of data with
no lost or marked DCCP-Ack packets, the Ack Ratio is decreased by 1,
again subject to the constraints on the Ack Ratio. See the section
below for the derivation. For a constant congestion window, this
gives an Ack sending rate that is roughly TCP-friendly. We note
that, because the sending rate for the acknowledgement packets
changes as a function of both the Ack Ratio and the congestion
window, the dynamics will be rather complex, and this Ack congestion
control mechanism is intended only to be very roughly TCP-friendly.
As a result of the constraints given earlier in this section, the
receiver always sends at least one ack packet for a congestion
window of one packet, and the receiver always sends at least two ack
packets per window of data otherwise. Thus, the receiver could be
sending two ack packets per window of data even in the face of very
heavy congestion on the reverse path. We would note, however, that
if congestion is sufficiently heavy that all of the ack packets are
dropped, then the sender falls back on a timeout, and the
exponential backoff of the timer, as in TCP. Thus, if congestion is
sufficiently heavy on the reverse path, then the sender reduces its
sending rate on the forward path, which reduces the rate on the
reverse path as well.
4.1.1. Derivation of Ack Ratio Decrease
The congestion avoidance phase of TCP increases cwnd by one MSS for
every congestion-free window. Applying this congestion avoidance
behavior to the ack traffic, this would correspond to increasing the
number of DCCP-Ack packets per window by one, after every
congestion-free window of DCCP-Ack packets. We cannot achieve this
exactly using the Ack Ratio, since the Ack Ratio is an integer.
Instead, we must decrease the Ack Ratio by one after K windows have
been sent without a congestion event on the reverse path, where K is
chosen so that the long-term number of DCCP-Ack packets per
congestion window is roughly TCP-friendly, following AIMD congestion
control.
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In CCID 2, K = (cwnd/(R^2 - R)), where R is the current Ack Ratio.
This result was calculated as follows:
R = Ack Ratio = # data packets / ack packets, and
W = Congestion Window = # data packets / window, so
W/R = # ack packets / window.
Requirement: Increase W/R by 1 per congestion-free window.
But can only reduce R by increments of one.
Therefore, find K so that, after K congestion-free windows,
the adjusted W/R would equal W/(R-1).
(W/R) + K = W/(R-1), so
K = W/(R-1) - W/R = W/(R^2 - R).
4.2. Quiescence
This section refers to quiescence in the DCCP sense (see section 6.1
of [DCCP]): How does a CCID 2 receiver determine that the
corresponding sender is not sending any data?
The receiver detects that the sender has gone quiescent after two of
its Ack Vectors are acknowledged without receiving any additional
data. That is, once the sender acknowledges two of the receiver's
Ack Vectors without sending additional data, the receiver can
determine that the sender is quiescent.
4.3. Acknowledgements of Acknowledgements
The sender, DCCP A, must occasionally acknowledge the receiver's
acknowledgements, so that the receiver can free up Ack Vector state.
The sender can also send acknowledgements to make changes to the Ack
Ratio. We assume that DCCP A manages the Ack Ratio proactively,
sending Change(Ack Ratio) options whenever required. To let the
receiver free Ack Vector state, DCCP A must occasionally acknowledge
that it has received one of DCCP B's acknowledgements. When both
half-connections are active, this information is automatically
contained in A's acknowledgements to B's data. If the B-to-A half-
connection goes quiescent, however, DCCP A must do it proactively.
In particular, the sender must acknowledge at least one of the
receiver's acknowledgements per congestion window, probably by
sending a DCCP-DataAck packet for the next datagram it sends. No
acknowledgement options are necessary, just the relevant
Acknowledgement Number in the DCCP-DataAck header. Of course, the
sender's application might fall silent before DCCP A can send an
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ack. This is no problem; A can wait arbitrarily long before sending
the ack.
5. Explicit Congestion Notification
ECN may be used with CCID 2. If ECN is used, then the ECN Nonce
will automatically be used for the data packets, following the
specification for the ECN Nonce in TCP in [SWE01]. For the data
subflow, the sender sets either the ECT(0) or ECT(1) codepoint on
DCCP-Data packets. Information about marked packets is returned in
the Ack Vector. Because the information in the Ack Vector is
reliably transferred, DCCP does not need the TCP flags of ECN-Echo
and Congestion Window Reduced.
For unmarked data packets, the receiver computes the ECN Nonce as in
[SWE01], and returns the ECN Nonce in DCCP-Ack packets. The sender
uses the ECN Nonce to protect against the accidental or malicious
concealment of marked packets.
Because the ack subflow is congestion-controlled, ECN can also be
used for DCCP-Ack packets. In this case we do not use the ECN
Nonce, because it would not be easy to provide protection against
the concealment of marked ack packets by the sender.
6. Relevant Options and Features
DCCP's Ack Vector option and Ack Ratio and Use Ack Vector features
are relevant for CCID 2.
7. Application Requirements
There are no specific application requirements for TCP-like
Congestion Control.
8. Thanks
We thank Mark Handley and Jitendra Padhye for their help in defining
CCID 2.
9. References
[DCCP] Eddie Kohler, Mark Handley, Sally Floyd, and Jitendra Padhye.
Datagram Congestion Control Protocol (DCCP). Work in progress.
[RFC 2026] S. Bradner. The Internet Standards Process -- Revision 3.
RFC 2026.
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[RFC 2861] M. Handley, J. Padhye, and S. Floyd. TCP Congestion
Window Validation. RFC 2861.
[SWE01] Neil Spring, David Wetherall, and David Ely. Robust ECN
Signaling with Nonces. draft-ietf-tsvwg-tcp-nonce-02.txt, work
in progress, October 2001.
10. Authors' Addresses
Sally Floyd <floyd@icir.org>
Eddie Kohler <kohler@icir.org>
ICSI Center for Internet Research,
1947 Center Street, Suite 600
Berkeley, CA 94704.
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